As the prevalence of childhood obesity has risen worldwide, so have the risks of associated medical conditions, such as type 2 diabetes and cardiovascular disease. To date, strategies to treat childhood obesity remain largely ineffective. Epidemiological studies have shown that patterns of increased food intake and adiposity in overweight children are predictive of adult obesity, and thus lend urgency to the need for novel approaches to combat the [unreadable]obesity epidemic[unreadable] in children. Research efforts in the past several decades have identified many signals and cellular components of neuronal circuits that regulate food intake and body weight;however, the vast majority of these studies have been performed in mature animals. Our analyses of physiological functions in LeprHYP KO mice, in which leptin signaling is disrupted in the hypothalamus, revealed a dramatic temporal shift in the progression of phenotypes related to food intake, adiposity and energy expenditure. Whereas young LeprHYP KO mice exhibited phenotypes similar to mice with systemic loss of leptin signals, with maturity, energy expenditure was normalized and food intake and adiposity levels stabilized. These observations are consistent with the following hypotheses: 1) regulation of phenotypes related to energy homeostasis may be different (and less complex) in immature animals;and 2) baseline levels of food intake and adiposity established in young animals may be maintained in adults. If true, those parts of the circuit that are critical in young animals would represent promising targets for novel strategies to combat childhood obesity. The goals of these studies are to define the roles of leptin-sensing neurons in the hypothalamus in establishing [unreadable]set-points[unreadable] for food intake and adiposity in young animals (Aim 1), and to examine how additional signaling networks interact to maintain energy homeostasis in mature animals (Aims 2 and 3). We crossed the Nkx2.1-Cre driver line to mice homozygous for a [unreadable]floxed[unreadable] allele of Lepr that disrupted leptin signaling in Cre-expressing cells in the hypothalamus, without affecting other leptin-sensing sites in the brain. The resulting LeprHYP KO mice are hyperphagic and obese. We will use a pair-feeding strategy to ascertain which phenotypes in LeprHYP mice are direct results of deficits in leptin signaling versus indirect effects of increased food intake (Aim1, Exp. 1). Introduction of ad libitum feeding to LeprHYP mice that had been previously pair-fed will be used to explore whether baseline levels of food intake and adiposity are established during a [unreadable]critical period[unreadable] in young animals and if so, whether they can be reset by restricting energy intake (Aim 1, Exp. 2). A similar genetic strategy will be used to disrupt insulin signaling in the hypothalamus (InsrHYP KO, Aim 2) and leptin signaling in the caudal brainstem (LeprNTS KO, Aim 3). Comparisons of physiological phenotypes in compound versus single KOs should provide insight into how these circuits interact with hypothalamic leptin signals to maintain energy balance.